Amide-imide polymers and copolymers are a relatively new class of organic compounds known for their solubility in nitrogen-containing organic solvents when in the largely polyamide form. In the past, the major application of these amide-imide polymers has been as wire enamels. This is illustrated in U.S. Pat. Nos. 3,661,832 (1972), 3,494,890 (1970) and 3,347,828 (1967). Amide-imide polymers and copolymers have also been found useful for molding applications as shown in U.S. Pat. Nos. 4,016,140 (1977) and 3,573,260 (1971). U.S. Pat. No. 4,136,085 (1979), U.S. Pat. No. 4,313,868 (1982), and U.S. Pat. No. 4,309,528 (1982) are incorporated herein by reference. These polyamides-imides are known for their outstanding mechanical properties; but they are also difficult to process and it is particularly difficult to form laminates from them. This difficulty is a consequence of insufficient flow of the polymer. The art has been looking for improvements in the flow and reduction in melt reactivity during fabrication of the polymers, but it is essential that an additive not impair the excellent mechanical properties of the amide-imide polymers and copolymers, when forming the laminate.
The general object of this invention is to provide amide-imide/amide-imide-phthalamide copolymers as the matrix resin for fiber laminates. Another object is to provide polyamide-imide/polyetherimide blends as the matrix reinforcer for fiber laminates wherein the polyethermide moiety is about 0.1 to about 50 percent. A more specific object of this invention is to provide polyamide-imide and polyamide-imide phthalamide impregnated woven fiber laminates, wherein the polyamide-imide moiety is about 95 to about 20 weight percent and the polyamide-imide-phthalamide moiety is about 5 to about 80 weight percent of the total coating added. Other objects appear hereinafter.
We have now found that amide-imide polymers and copolymers obtained by reacting a polycarboxylic acid anhydride with one primary diamine or a mixture of primary diamines containing about 5 to about 80 percent by weight of polyamide-imide-phthalamide moieties produce excellent impregnation resins for fiber laminates. When polyamide-imide alone is used, higher molding pressures are required. Suitable polyamide-imide phthalamides which can be used with polyamide-imides as impregnating resins for fiber laminates comprise recurring polyamide A units of: ##STR1## which are capable of undergoing imidization and polyamide B units of: ##STR2## wherein the molar ratio of A units to B units is between about 80 to 20 and about 20 to 80, wherein R is a divalent aromatic hydrocarbon radical of from about 6 to about 20 carbon atoms or two divalent hydrocarbons joined directly or by stable linkages selected from the group consisting of --O--, methylene, --CO--, and --SO.sub.2 --, and wherein X is a divalent aromatic radical and T denotes isomerization.
In the injection molded form, the polyamide A units have converted to the polyamide-imide A' units and the copolymer is comprised of recurring polyamide-imide A' units of: ##STR3## and polyamide B' units of: ##STR4## wherein the molar ratio of A' to B' units is about 80 to about 20 to about 20 to about 80, preferably about 1 to about 1, and wherein R and X are defined as above.
The polyamide-imide phthalamides are prepared from acyl halide derivatives of dicarboxylic acid, such as isophthalic acid or terephthalic acid and an anhydride-containing substance and aromatic diamines. Useful acyl halide derivatives of dicarboxylic acid include: ##STR5## and related compounds. Suitably, the anhydride-containing substance is an acyl halide derivative of the acid anhydride having a single benzene or lower-acyl-substituted benzene ring. The preferred anhydride is the four-acid chloride of trimellitic anhydride (4-TMAC).
Useful aromatic diamines include para- and metaphenylenediamine, oxybis (aniline), thiobis (aniline), sulfonylbis (aniline), diaminobenzophenone, methylenebis (aniline), benzidine, 1,5-diaminoaphthalene, oxybis (2-methylaniline), thiobis (2-methylaniline), 2,2-bis-4-(p-aminophenoxy)phenylpropane, bis-4-(p-aminophenoxy)phenylsuflone, 2,2-bis-4-(p-aminophenoxy)phenylhexafluoropropane, bis-4-(p-aminophenoxy)benzene, bis-4-(3-aminophenoxy)benzene, and the like. Examples of other useful aromatic primary diamines are set out in U.S. Pat. No. 3,494,890 (1970) and U.S. Pat. No. 4,016,140 (1977) both incorporated herein by reference. The preferred diamine is methaphenylenediamine.
The amount of the polyamide-imide-phthalamide added to the polyamide-imide polymer can be about 5.0 to about 80 weight percent, usually in the range of about 10 to about 20 weight percent. The polyamide-imide-phthalamide is miscible in our amide-imide polymers, thus forming a single glass transition (Tg) matrix. When about 20 weight percent of the polyamide-imide-phthalamide was dry blended with out amide-imide polymer and molded, a single Tg was found. The Tg of our amide-imide polymer used as a control was about 230.degree. C. to about 240.degree. C., as molded, while the polymer containing 20 percent by weight of polyamide-imide-phthalamide also had a glass transition temperature of about 230.degree. C. to about 240.degree. C. After being cured at a temperature of about 160.degree. C. to about 260.degree. C., the glass transition temperature for our control polyamide-imide polymer rose to about 270.degree. C. and for the sample containing 20 percent polyamide-imide-phthalamide the glass transition temperature also rose to 270.degree. C.
It should be particularly emphasized that when our polyamide-imides are blended with polyamide-imide-phthalamide, a one-phase miscible polyamide-imide/polyamide-imide-phthalamide system is obtained. This is critical in the effectiveness of our process and our novel laminate compositions, since if a one-phase miscible system is not formed, delamination of the incompatible components can readily occur with a multiphase polymer system.
The polyamide-imides are prepared by reacting an acyl halide derivative of an aromatic tricarboxylic-acid-anhydride with one or a mixture of largely- or wholly-aromatic primary diamines. The resulting products are polyamides wherein the linking groups are predominantly amide groups, although some may be imide groups, and wherein the structure contains free carboxylic acid groups which are capable of further reaction. Such polyamides are moderate molecular weight (3000-13,000 as prepared) polymeric compounds having, in their molecule, units of: ##STR6## wherein the free carboxyl groups are ortho to one amide group, Z is an aromatic moiety containing 1 to 4 benzene rings or lower-alkyl-substituted benzene rings; R.sub.1, R.sub.2 and R.sub.3 are the same for homopolymers and are different for copolymers and are divalent wholly- or largely-aromatic hydrocarbon radicals. These hydrocarbon radicals may be a divalent aromatic hydrocarbon radical of from 6 to about 10 carbon atoms, or two divalent aromatic hydrocarbon radicals each of from 6 to about 10 carbon atoms joined directly or by stable linkages such as --O--, methylene, --CO--, --SO.sub.2 --, or --S--; for example, --R'--O--R'--, --R'--CH.sub.2 --R'--, --R'--CO--R'--, --R'--SO.sub.2 --R'-- and --R'--S--R'--.
Said polyamides are capable of substantially complete imidization by heating, by which they form the polyamide-imide structure having, to a substantial extent, recurring units of: ##STR7## wherein one carbonyl group is meta to and one carbonyl group is para to each amide group and wherein Z, R.sub.1, R.sub.2 and R.sub.3 are defined as above. Typical copolymers of this invention have up to about 50 percent imidization prior to heat treatment, typically about 10 to about 40 percent.
We can use a single diamine but, usefully, the mixture of diamines contains two or more, preferably two or three, wholly- or largely-aromatic primary diamines. More particularly, they are wholly- or largely-aromatic primary diamines containing from 6 to about 10 carbon atoms or wholly- or largely-aromatic primary diamines composed of two divalent aromatic moieties of from 6 to about 10 carbon atoms, each moiety containing one primary amine group, and the moieties linked directly or through, for example, a bridging --O--, --S--, --SO.sub.2 --, --CO--, or methylene group. When three diamines are used, they are preferably selected from the class composed of: ##STR8## said X being an --O--, --CH.sub.2 --, or --SO.sub.2 -- group. More preferably, the mixture of aromatic primary diamines is in the one-component or two-component system and is composed of meta-phenylenediamine, p,p'-oxybis(aniline) and meta-phenylenediamine, or p,p'-sulfonylbis(aniline) and p,p'-methylenebis(aniline). Most preferably, the mixture of primary aromatic diamines contains meta-phenylenediamine and p,p'-oxybis(aniline). In the one-component system, the preferred diamines are oxybis(aniline) or meta-phenylenediamine. The aromatic nature of the diamines provides the excellent thermal properties of the copolymers while the primary amine groups permit the desired imide rings and amide linkages to be formed.
Usually, the polymerization or copolymerization is carried out in the presence of a nitrogen-containing organic polar solvent such as N-methylpyrrolidone, N,N-dimethylformamide and N,N-dimethylacetamide. The reaction should be carried out under substantially anhydrous conditions and at a temperature below about 150.degree. C. Most advantageously, the reaction is carried out from about 20.degree. C. to about 50.degree. C.
The reaction time is not critical and depends primarily on the reaction temperature. It may vary from about 1 to about 24 hours, with about 2 to 4 hours at about 30.degree. C. to about 50.degree. C. being preferred for the nitrogen-containing solvents.
We also have found that if about 0.1 to about 50 percent of the polyamide-imide polymer is replaced with about 0.1 to about 50 percent by weight of polyetherimide, the resulting polymer composition is a superior matrix resin for fiber laminates and particularly for carbon fiber laminates.
Suitable polyetherimides comprise essentially chemically combined units of the formula: ##STR9## where R is a member selected from the class consisting of (a) the following divalent organic radicals: ##STR10## and (b) divalent organic radicals of the general formula: ##STR11## where X is --C.sub.y H.sub.2y --, y is a whole number equal to 1 to 5 inclusive, and R.sup.1 is a divalent organic radical selected from the class consisting of (a) aromatic hydrocarbon radicals having from 6-20 carbon atoms and halogenated derivatives thereof, (b) alkylene radicals and cycloalkylene radicals having from 2-20 carbon atoms, (c) C.sub.(2-8) alkylene terminated polydiorganosiloxanes, and (d) divalent radicals included by the formula: ##STR12## where Q is a member selected from the class consisting of: ##STR13## and x is a whole number equal to 1 to 5, inclusive.
We have unexpectedly discovered that blends of polyamide-imide polymers and polyethermides, disclosed herein, range can be made over a wide range in which the properties of the blend show a marked average improvement over the properties of the components of these blends. The improvements in properties of the blends are unexpected to a person skilled in the art, considering the proportion of either the polyamide-imide or the polyetherimide used. In our novel blends, synergistic results are obtained which are not characteristic of either blend and the application for our blends is of a much greater range than for the unblended material. In addition, by blending the polyamide-imide with polyetherimides, products can be obtained which are lower in cost than products which are usually produced by the use of the polyamide-imide alone without significant sacrifice, if any, in thermal properties.
A preferred class of polyethermidies which is included by formula (III) consists of polymers comprising of from about 2 to 5000 or more units and, preferably, from about 5 to about 100 units of the formula: ##STR14## where R.sup.1 is as previously defined, and R.sup.2 is: ##STR15##
Included by the polyetherimides of formula III are polymers comprising the following chemically combined units: ##STR16## and mixtures thereof, where R.sup.1 and R.sup.2 are as defined above.
The polyetherimides of formulas III-IV can be made by effecting reaction between an aromatic bis(etheranhydride) of the general formula: ##STR17## and an organic diamine of the general formula: EQU H.sub.2 NR.sup.1 NH.sub.2 (VIII)
where R and R.sup.1 are as previously defined.
There can be employed from 0.95 to 1.05 mols of aromatic bis(etheranhydride) per mol of organic diamine.
In making the polyetherimides, there are employed from 0.95 to 1.05 mols of the aromatic dianhydride of formula VII per mol of the organic diamine of formula VIII. Preferably, one can employ equal or lower amounts of the bisanhydride and diamine.
The aromatic bis(etheranhydride) of formula VII, shown in the above-mentioned U.S. Pat. No. 3,847,867, can be prepared from the hydrolysis followed by dehydration of the reaction product of the nitro-substituted phenyl dinitrile and then continuation of the reaction with a dialkali metal salt of a dihydric aryl compound in the presence of a dipolar aprotic solvent, where the alkali metal salt has the general formula: EQU Alk--O--R.sup.1 --O--Alk
where R.sup.1 has the meaning given above and preferably is the same as R.sup.2 and Alk is an alkali metal ion. Various well-known procedures can be used to convert the resulting tetranitriles to the corresponding tetraacids and dianhydrides.
The amount of the polyetherimide added to the polyamide-imide polymer can be about 0.1 to about 50 weight percent, usually in the range of about 2 to about 20 weight percent.
Laminates of amide-imide copolymer solution-impregnated fiber woven fabric have been produced at lower molding pressures when about 5 to about 80 percent by weight of the polyamide-imide-phthalamide is added to the impregnation solution.
High performance composites are usually made by lamination, i.e. the fixing together of sheets of aligned fiber reinforced polymers. The fiber directions used are chosen to suit the magnitudes and directions of the stresses that are expected to be encountered. The alignment of these reinforcement fibers can be in the same direction in unidirectional layups such as unitape and unidirectional fabric. Additionally, each successive layer of the laminate can have different fiber direction from the previous one, except for the two layers at the center. Typically, the fiber direction in each successive layer of the laminate can be shifted, for example, by 30.degree., 45.degree., or 90.degree. from the previous layer. Usually the layers are "balanced", i.e. they consist of an even number of sheets, arranged so that the interface between the two sheets at the center is a mirror plane of symmetry. This is to avoid unwanted twisting and other distortions which occur with unbalanced laminates when the laminate is stressed. Very high volume fractions of reinforcement can be obtained in laminates, and this is much the most efficient way of providing bi-directional or approximately transversely isotropic reinforcement. The fiber used for reinforcement can be any material which can be processed as a continuous filament and has a modulus of 10,000,000 psi or greater and is thermally stable to at least 600.degree. F. for at least 10 minutes. By thermally stable is meant the fiber emits insufficient volatiles to cause voids in the final composite structure. While any high temperature stable fiber material can be used according to the present invention, such as glass fibers, alumina, steel, silicon nitride, silicon carbide, boron, Kevlar, carbon fiber or the like.
The term carbon fiber is used herein in the generic sense and includes graphite fibers as well as amorphous carbon fibers which result after a thermal carbonization or graphitization treatment. Graphite fibers are defined herein to consist substantially of carbon and have a predominant X-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit a predominantly amorphous X-ray diffraction. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
The reinforcement fibers are coated or sized with a polyamic acid, an amide-imide polymer, an amide-imide copolymer or mixtures of these materials (hereinafter collectively referred to as amide-imide polymers or alternatively polyamide-imide). These materials are prepared from an anhydride-containing substance and a mixture of wholly- or partially-aromatic primary diamines or fully or partially acylated diamines. The amide-imide polymer sizing agents can be applied to the fiber in a suitable solvent, which is non-reactive with the sizing agent, to control the amount of size coated onto the fiber. The presence of solvent will improve the ability of the sizing agent to penetrate into the individual fibers of a staple yarn, filament yarn, or roving. The concentration of the size in the solvent is usually in the range of from about 0.05 to about 10%, and preferably from about 0.5 to about 5% by weight, based on the total weight of the solution. Examples of suitable solvents are N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC) and methyl ethyl ketone. Other materials to aid in the removal of the solvent such as methylene chloride and the like can be added to the solvent.
The reinforcement fibers sized with amide-imide polymers are used to form reinforcement material for continuous fiber, unidirectional tape, and woven fabric for which amide-imide polymers are used as the matrix resin. These amide-imide polymers may be modified with other polymeric materials to improve the flow properties of the polymer matrix during molding or consolidation. Examples of these modifier polymers are amorphous and semi-crystalline polyamides or polyetherimides.
Amorphous or semi-crystalline polyamides have also been found to aid the manufacture of amide-imide impregnated carbon woven fiber laminates and chopped fiber molding compounds. Suitable amorphous polyamides have both aromatic and aliphatic moieties. Advantageously, the amorphous polyamide comprises recurring units of the following structure: ##STR18## wherein Y is a straight chain of one to six methylene groups, said chain being substituted by at least one alkyl group, the total number of side chain carbon atoms introduced by the alkyl substitution being at least one.
Another amorphous polyamide group suitable for use in improving the melt flow and reducing the melt reactivity of our amide-imide polymer has the following structure: ##STR19## Amorphous polyamides of the following structure are preferred for use in our process to modify the polyamide-imides to improve flow during the consolidation phase of the laminate formation.
The amorphous polyamide, Trogamid-T, manufactured by the Dynamit Nobel Company, has the following structure and is particularly useful in improving the flow properties and reducing the melt reactivity of the polyamide-imide: ##STR20## wherein X is CH.sub.2.
Another very useful amorphous polyamide is Amidel, manufactured by Union Carbide Company and having the following structure: ##STR21## more particularly wherein the first X is (CH.sub.2).sub.7, the second X is CH.sub.2, and the third X is (CH.sub.2).sub.4.
Other useful polyamides include the Upjohn amorphous polyamide of the following structure: ##STR22## wherein the first X is (CH.sub.2).sub.9 and the second X is CH.sub.2, and the copolyamide of the following structure: ##STR23## wherein X is (CH.sub.2).sub.6.
In all of the foregoing structures X can be a straight chain of one to five CH.sub.2 groups. X can be the same or different in each amorphous polyamide moiety. Some of the semi-crystalline polyamides which increase the amide-imide polymer flow properties without significantly altering the glass transition temperature of the amide-imides polymers are: nylon 6/6, nylon 6, nylon 6/12, nylon 11, nylon 12, etc. The amorphous polyamides are ordinarily used in quantities ranging from about 0.1-20 weight percent while the semi-crystalline materials are used in amounts of about 0.1-5 weight percent.
Unidirectional tape (unitape) formed by parallel reinforcement fiber bundles forming tape widths of 0.25 inch to 24 inch can be impregnated with polyamide-imide matrix resin in the following manner. The unitape is placed on release paper on a table or platen heated to a temperature of 100.degree. to 200.degree. F. The release paper can be polyethylene coated paper. A concentrated solution of polyamide-imide in a suitable solvent such as NMP is pressed into the reinforcement fibers of the unitape and worked into the tape to provide uniform wetting and impregnation of the reinforcement fibers. The solution contains a solids concentration of 45 to 70% polyamide-imide in the solvent. A second release paper can be placed on top of the unitape after the matrix resin solution has been added before the unitape is pressure rolled. The solvent is removed by heating the unitape to a temperature of 300.degree.-500.degree. F. The unitape is cooled to room temperature and rolled up on a roll for shipment having a residual solvent content of 1 to 10%.
For single fiber or multiple fiber bundles up to 0.25 inch in diameter, these continuous fibers are impregnated with polyamide-imide by dip coating the continuous filament in a dip bath containing the impregnation material. The dip bath can be operated at room temperature to 200.degree. F. with a solids content of 25 to 40% polyamide-imide in a suitable solvent. The continuous filament is dried at temperatures of 300.degree. to 500.degree. F. and wound to form packages for use in spiral winding, pultrusion, and the like. The residual solvent content is about 1 to 10%.
The term "composite structure" is defined herein as fiber reinforced fabric, tape or broadgoods which has been impregnated with amide-imide polymer and is also referred to herein as "prepreg". In a batch operation, an appropriate quantity of the reinforcement fiber is spread in a parallel lay-up on a flat surface to the thickness and width needed. A measured quantity of resin is added to the fibers. The resin may be in a solvent, to improve uniformity of impregnation, with the solvent removed after coating.
In a continuous operation, the fibers, tapes, or fabrics can be spread and resin added by passing over and through a series of appropriately spaced rolls above and within a container of the matrix resin or resin/solvent mixture. The solvent is removed by passage through a heating zone at a temperature sufficient to evaporate the solvent. Otherwise, the fibers can be impregnated by the so called melt transfer technique, where the resin is transferred to the fiber by contact with a moving belt containing the resin at a temperature high enough to maintain the resin in a plastic state without the addition of solvent.
Laminates of the desired thickness are formed by using multiple layers of prepreg and subsequently placing the layers under sufficient temperature and pressure to form a substantially void free laminate.
In a preferred method, a carbon fiber woven fabric for prepreg preparation, is formed by impregnating carbon fiber woven fabric with amide-imide polymer. The carbon fiber used to form the fabric is sized with amide-imide polymer to aid in the adhesion of the carbon fiber to the matrix resin. The fabric is drawn through a dip tank which contains a 25 to 40% solution of the amide-imide polymer in NMP solvent at a temperature in the range of room temperature to 200.degree. F. The woven fabric must have sufficient residence time in the dip tank provided with numerous rollers to completely "wet-out" the fabric and provide for complete impregnation of the fabric with matrix resin. As the fabric exits the dip tank it must pass through nip rolls to control the solution/dry polymer content on the fabric. The dry resin content of the fabric is in the range of 30 to 50 weight percent based on the fabric. The prepreg is dried at 300.degree. to 500.degree. F. to give a residual solvent content of 1 to 10 weight percent.
Dried prepreg is cut to the desired dimensions and placed on a metal place which each successive layer tacked together in some manner to form a lay-up. The number of layers or plys depends on the desired thickness of the laminate. Release and breather fabrics are placed on top of the lay-up with the breather fabric extending beyond the edges of the prepreg. Material to form a vacuum bag is placed over the lay-up and a vacuum bag is formed. The entire assembly of the lay-up in a vacuum bag is placed in a hydraulic press or a vacuum-bag autoclave. After forming a vacuum in the bag, the assembly is heated to the range of 660.degree. F., allowed to equilibrate, and a pressure of 100 to 500 psig is applied for about 5 to 30 minutes. The assembly is allowed to cool under pressure before the laminate is removed from the assembly.
The following examples illustrate the preferred embodiments of the invention. It will be understood that the examples are for illustrative purposes only and do not purport to be wholly definitive with respect to conditions or scope of the invention.